P3D Re: Hollow Face Illusion

[Bruce Springsteen <bsspringsteen@xxxxxxxxx>]

George Themelis wrote:

> Bruce, was the concave face that you saw lighted from the
> bottom?  Light from the bottom reinforces the illusion.

No, from above in a general diffuse fluorescent light.

> How far away were you standing from it?  The further you stand
> the stronger it is (since your depth perception is reduced).

It was about a foot tall, and I was five or six feet away.  I suppose if I
stood far enough away, there would be no stereopsis at all, but the rules
didn't stipulate a minimum viewing distance for experiencing the
inversion. ;-) 
> For me it is so strong that many times I have to touch it to
> go back to normal.  Maybe the construction is important too.
> My "face" is totally white. With light from the bottom it
> is only white or black (shadows).  Nothing to help you form
> a good stereo image.

I really hope some of you will find the time to read the references to
this illusion in the R. L Gregory book "The Intelligent Eye" (US
publication - 1970, McGraw-Hill, Inc.) and also the article "Binocular
Depth Inversion" by John I. Yellett, Jr. in Scientific American, July
1981.

Gregory treats the hollow mask as a strong example of his general subject,
how the mind forms a perceptual hypothesis of what real object is being
sensed, even overriding the sensory input if it contradicts what the mind
expects to see.  He mentions the "following" aspect, but is more
interested in the depth inversion of the concave object, which allows the
parallax illusion to occur.  He has several full-page anaglyphs of a
hollow mask to illustrate this with his brief treatment.

Yellott reports that it was Gregory's comments that drew his attention to
the subject.  He mentions the "following" business, referring to the
Haunted Mansion at Disneyland, but is like Gregory much more interested in
the "perceptual mistake" of the perceived depth inversion.  He describes a
series of experiments to gauge the strength of this tendency (extremely
strong) and some speculation as to how the mind can simultaneously see and
reconcile the contradiction between stereoscopic input and perceived
depth.

This article came to my attention a year ago when I found it in the same
issue on anamorphic photography, which was my P3D topic at topic then. 
Mentioned this article as a sidebar.  Now is is the foreground subject,
and the anamorph piece is receeded - a reversal of figure/ground! ;-)

But the most pleasing item is the cover of this SciAm issue, which is to
illustrate a piece about Christopher Wren (British architect).  It is a
drwawing of two alcoves that, when cross-viewed, happen to make a very
nice accidental stereo pair.  And when parallel-viewed, I find that I can,
by changing my image hypothosis, oscillate between perceiving
pseudo-stereo and ignoring the stereo cues to see the fused alcove in
correct relief - by act of will alone!  A serendipitous practice image for
the very issue at hand!  Try it, you'll like it!

Bruce 


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